The present invention relates to an injection compression molding method and an apparatus therefor, which are capable of accurately controlling a compression force applied to molten resin injected into a mold of an electrically-operated injection compression molding machine so as to uniformly fill the mold with the molten resin. This produces high-quality molded products with high reproducibility.
It is known to effect injection compression molding wherein a molten resin injected into a mold is compressed, so that the molten resin reaches fine parts of a cavity of the mold. This attains improvement in dimensional accuracy and uniformity of density of molded products. Typically, injection compression molding is effected by driving a compression pin, provided in a clamping mechanism of a hydraulic injection molding machine, from a movable platen toward the mold by the use of a hydraulic mechanism, so that the compression pin applies a compression force on the molten resin in the mold. However, according to the prior art apparatus of the type driving the compression pin by means of the hydraulic mechanism, due to the presence of variations in properties of hydraulic oil for the hydraulic mechanism resulting from a change in temperature of the injection molding machine, it is difficult to control the driving force applied to the compression pin and the moving position of the compression pin. That is, the compression force applied to the molten resin with required accuracy and reproducibility is not attained. This results in a variation of quality between the resultant molded products.